1. Field of the Invention
The present invention relates to a method for forming a phosphor screen for a color cathode-ray tube and an exposure apparatus, and more particularly to a method for forming a black matrix between phosphor dots and an exposure apparatus.
2. Description of the Related Art
The phosphor screen of a color cathode-ray tube is constituted by phosphor dots having three luminescent colors and coated on the inner surface of a face panel, and a black material (black matrix) embedded between the phosphor dots and irrelevant to light emission.
In general, a method of manufacturing the phosphor screen mainly includes a black matrix forming step and a phosphor dot forming step, and employs a printing method using a photoresist.
Specifically, in the black matrix forming step, a polyvinyl alcohol (PVA) containing a photosensitive material, which is hardened when an ultraviolet ray is applied thereto, is coated on the inner surface of a panel to form a photoresist film. Then, an exposure light source is set in a position corresponding to the position from which an electron beam of each color is to be emitted, and a light beam is emitted from the source onto the photoresist film through a shadow mask opposed to the inner surface of the panel. As a result, predetermined portions of the photoresist film corresponding to the electron beam apertures in the shadow mask, i.e., those portions on which phosphor dots are formed, are exposed to the light beam. After the exposure step, non-exposed portions are removed from the photoresist film, thereby forming a resist pattern. Subsequently, a black material is coated on the resist pattern, and an oxidizer is injected onto the inner surface of the panel to decompose the resist. The resist and an unnecessary portion of the black material are removed by spraying water with high pressure, thereby forming a black matrix with holes for forming phosphor dots therein.
In the phosphor dot forming step, a slurry consisting of a photosensitive PVA liquid and phosphor particles dispersed therein is coated on the black matrix on the panel inner surface, and only those portions of the slurry which correspond to the holes of the black matrix are exposed to light with the use of a shadow mask, as in the above-described exposure step, thereby attaching phosphor thereto, and removing the other portions by spraying water with high pressure. This step is repeated for forming phosphor dots of each color.
An exposure apparatus to be used in the above-described exposure step generally has a frame for supporting the panel on which the black matrix and the phosphor dots are to be formed, and the shadow mask located on the inner side of the panel; an exposure light source for emitting light onto the inner surface of the panel with the shadow mask interposed therebetween and a correction lens provided between the exposure light source and the shadow mask, for causing the path of light from the exposure light source to approach the path of an electron beam.
The light from the exposure light source is restricted through circular electron beam apertures in the shadow mask, forming substantially circular exposed portions in the resist film on the inner surface of the panel, and forming a black matrix in the same manner as described above. Each hole of the black matrix has the same shape as the cross section of the bundle of the exposure light rays radiated onto the panel.
In the case of a color cathode-ray tube for a very high-resolution display, which has a shadow mask with apertures arranged with a small pitch, it is preferable to form the shadow mask thick, in order to keep a sufficient mechanical strength of the shadow mask, in light of manufacturing the tube. Each aperture of the shadow mask is generally defined by a boundary portion between a smaller opening formed in the surface of the shadow mask facing the electron gun and a larger opening formed in the surface of the mask facing the phosphor screen. The smaller opening is made to have a predetermined transmittance. In order to keep the strength of the shadow mask at a desired value, however, there is a case where the larger opening cannot have a sufficient size. For this reason, the exposure light beam to be applied to that part of the black matrix which is located in a peripheral portion of the phosphor screen is influenced not only by the aperture defined by the boundary portion between the larger and smaller openings, but also by the smaller and larger openings themselves.
As a result, in the peripheral portion of the phosphor screen, part of a hole formed in the black matrix is deformed to have the shape of an elliptic. Since the shape of the holes in the black matrix corresponds to that of phosphor dots, a non-circular phosphor dot is created, thereby reducing the light output of the color cathode-ray tube.
To solve the above problem, there has been proposed a method for improving an aperture in the shadow mask to have the shape of an ellipse whose major axis extends in a radial direction; or a method for moving a light source in the direction of the tube axis at the time of exposing the photoresist film (Jpn. Pat. Appln. KOKAI Publication No. 62-17925).
However, in the method for improving the apertures of the shadow mask to have the shape of an ellipse whose major axis extends in a radial direction, an area of the remaining portion of the shadow mask is reduced and hence the strength of the mask is reduced. Further, in the method for moving a light source in the direction of the tube axis at the time of exposing the photoresist film, the exposure unit inevitably has a complicated structure. Especially, in the case of using a rotary light source in this method, the exposure unit is much more complicated, and therefore the accuracy of assembly of the unit is reduced, degrading the quality of the color cathode-ray tube.